State of the art determination of prescription for an individual generally includes a two step process. First a start value for each eye is captured objectively with an auto-refractor. Each eye is measured consecutively while the patient is fixating a target inside the device. The results are then transferred into a trial frame or phoropter head and refraction is fine-tuned based on subjective feedback from the patient in a second separate step. For a complete eye examination of visual performance, additional tests such as pupil distance, heterophoria, binocular balance or near-addition tests etc. are performed during subjective refraction. These tests enable the practitioner, with the subjective information from the patient, to select a visual aid (e.g. progressive spectacle lenses, simultaneous vision bifocal contact lenses, intraocular lenses, LASIK procedures etc.).
Several factors are responsible for the limited accuracy of auto-refractors determining prescription. A well known issue is instrument myopia caused by the close proximity of the device in front of the eye despite target projection into infinity by optical means. Lack of control of focus and vergence of the non-measured eye and other factors add to the instrument myopia problem. Some auto-refractors try to overcome these problems by providing open space natural binocular viewing conditions while measuring refraction of the eye. Although their results show better correlation they are still not always identical to subjective refraction. There are further significant reasons for this, for example the refraction found monocularly for each eye can change when the eyes are operating together (hence the requirement for binocular balance measurements) and there may be cyclorotation of the eyes which will affect cylinder angle. Additional but required tests to provide prism correction cannot be performed monocularly. There is also an element of patient acceptance when defining the correct prescription and subjective binocular refraction allows patients to express their preference. For these reasons subjective binocular refraction is considered the gold standard.
Wavefront sensors provide a more detailed objective picture of visual performance by measuring lower and higher order aberrations of the eye. Generally they encounter the same problems caused by unnatural viewing conditions during measurement as auto-refractors and similar measures to address them have been proposed. Recently introduced ocular aberration based techniques for prescribing visual aids appear to partially overcome the need for subjective refraction. Today, however, practitioners applying these new techniques still perform subjective refraction because sphere, cylinder and axis values, and the higher order corrections available with wavefront refraction, are not the only values that provide a complete prescription (see above).
Another typical step in the process that starts with visiting an eye doctor or optometrist and ends with wearing a satisfactory vision aid is the varying subjective tolerance of individuals to optical designs of vision aids, particularly to bifocal or multifocal designs. It is very difficult for the practitioner to predict how the patient's subjective tolerance to an optical design will be without actually putting the optical design in front of the patient's eyes. For example it is typically not advised to prescribe progressive lenses to someone who has worn bifocal lenses for many years: in such a case the likelihood is high that the patient will not get used to the progressive lens design, though the underlying reasons for this are not well understood. This issue becomes particularly critical when simultaneous vision intraocular lenses are implanted in a cataract surgery procedure. Not only is it not possible to put the design in front of the patient's eye before surgery, it is also not possible to remove the design easily post-surgery should the patient reject the visual performance of simultaneous vision. The possibility to get individual subjective tolerance feedback to an optical design before a visual aid is ordered and manufactured would be a significant asset for the practitioner's decision making process.
The relevance of individual subjective tolerance to optical designs will increase in the future. Therapeutic optical designs to control myopia are on the horizon. These new designs will fundamentally change the objective of optical prescriptions from pure symptom treatment methods to therapeutic prevention or curing methods of refractive errors. The two most important aspects of these procedures will be firstly the efficiency of the therapeutic treatment effect and secondly individual subjective tolerance to the therapeutic optical design. Again, the possibility to get individual subjective tolerance feedback to a therapeutic optical design before a visual aid is ordered and manufactured would be a substantial asset for the practitioner's decision making process.
U.S. Pat. No. 5,963,300 to Horwitz describes an ocular biometer for monocular and binocular interferometric wavefront and other measurement of the eye utilizing “hot mirrors” to allow the eye to view the world as the ocular biometer objectively measures various characteristics of the eye such as convergence via Purkinje images. Thus Horwitz, among others, enables strabismus measurements.
However, Horwitz fails to control and measure vergence by means of virtual images that can be optically separated for each eye and individually displaced either horizontally or vertically. Therefore no heterophoria measurements can be performed. Horwitz furthermore does not take into account subjective measurement procedures such as heterophoria, binocular balance, or subjective refraction feedback to be combined with objective measurements.
The major drawbacks of other measurement techniques for the determination of visual aids of the human eye are the unnatural viewing condition used by objective measurement methods (subsequent monocular measurement of each eye combined with instrument myopia) as well as the separation of objective and subjective measurement techniques into two work steps. While there are several objective measurement techniques that use open space viewing conditions (e.g. Canon, Wavefront Sciences) it is not possible to get subjective visual feedback from the patient to optimize or modify the result of these devices in any form and it is not possible to control vergence. Neither is it possible to simulate visual perception of visual correction methods for example induced by multi-focal contact lenses or spectacles.
Adaptive optics systems to measure and correct all aberrations of the eye have been described in various implementations, that can include an incorporated eye tracking system, binocular measurement, far and near distance measurement and photopic or mesopic viewing conditions. However none of these systems take into account the natural viewing conditions of the eye under binocular free space viewing that is required to fine tune both objective and subjective individual vision corrections.